Mr. Polecat,
previously, cartridges that headspaced on a rim( or belt for that matter) often had the shoulder area too far forward in the chamber. Such chambers would still pass a "go" gauge and not a "no go". Your problem could likely be addressed by fireforming cases with a small load of "fast" powder like Unique, 1/4 sheet tp, grits as filler, and more tp to close it up. Once fireformed, set the loading dies to just "kiss" the shoulder. Then only use your handloads.FWIW in a drilling, you won't be limited to round nose or flat bullets.
Mike